From EP-A-0 658 611, a shock absorber piston is known that is provided with a revolving web on its peripheral surface, a multiplicity of webs that run in the axial direction being adjacent to or connected to one side of this revolving web. Using an injection molding method, a thermoplastic plastic seal is sprayed onto this piston body, and in particular the webs extending in the axial direction act, with their intermediate grooves filled with sealing material, to reliably anchor the sealing material and to guide the piston. The sealing applied in the injection molding enables a narrow tolerance in order to avoid what is known as “blow-by” and thus to effect a reliable sealing of the mutually associated cylinder chambers. The method for manufacturing seals that are injection-molded in this way is relatively expensive.
From U.S. Pat. No. 3,212,411, a piston-cylinder arrangement is known whose piston body has a multiplicity of revolving grooves on its peripheral surface. In order to apply the seal, a cup-shaped preform made of PTFE (polytetrafluorethylene) is provided that is first placed loosely on the piston body. The piston body prepared in this way is subsequently first pressed into a highly heated shaping and calibrating cylinder, where the PTFE material is pressed into the grooves on the peripheral surface of the piston body under the influence of heat. Subsequently, the piston body with the pressed-on seal is cooled in a correspondingly fashioned cooling cylinder. The grooves are filled completely with the sealing material, in order to bring about a positive, fixed connection of the seal with the peripheral surface of the piston body. For use as a shock absorber piston, the base surface of the preform, still covering the final piston area or end surface of the piston body on one side, must subsequently be removed.
From EP-A-682 190, a shock absorber piston is known that differs in its manufacture from the above-described method essentially only in that a stamped annular disc is used to apply the seal, instead of a cup-shaped preform. This annular disc is placed on one end of the piston body. The piston body prepared in this way is again pressed into a heated shaping and calibration cylinder, and the annular disc is placed around the peripheral surface of the piston body as a band and is subsequently pressed into the grooves running in the peripheral direction of the piston body under the influence of heat. Subsequently, the piston, with its pressed-on seal, is guided through a cooling tube. Here as well, the sealing material fills the grooves practically completely, so that the sealing is connected in positive, fixed fashion with the peripheral surface of the piston body.
The two methods described above have the disadvantage that considerable pressure can be required for the deforming and the complete pressing of the sealing material into the grooves on the peripheral surface of the piston body, and that the sealing material that forms the seal is subjected to strong deformations that disadvantageously influence the structure of the sealing material.
From DE-A-198 47 342, a piston is known for a piston-cylinder arrangement, in particular a shock absorber piston, having a piston body that is provided, on its peripheral surface in an area adjacent to one end of the piston, with a revolving web that protrudes beyond the peripheral surface, to which longitudinal support webs are adjacent that run in the direction towards the other end of the piston, disposed in parallel to one another and at a distance from one another, each pair of adjacent support webs delimiting a groove-shaped recess that is open at its ends opposite the revolving web in the longitudinal direction, and having a collar-shaped seal made of a thermally deformable sealing material that is formed onto the piston body in such a way that both the revolving web and the support webs are formed into the material of the seal only over a part of their vertical direction or height.
This known solution has shown that for a good sealing between the collar-shaped seal of the piston on the one hand and the external contour of the piston body on the other hand, it is not required for the seal to lie tightly against the piston body over the full periphery. For many cases of application, it is sufficient if the sealing collar lies tightly only on the relatively narrow revolving web in the peripheral direction. In addition, it has turned out, surprisingly, that for a problem-free and reliable connection between the seal and the piston body it is not required that the groove-shaped recesses between the longitudinal support webs be filled completely by the sealing material. In this way, there remains a sufficient open space into which the sealing material can escape in case of expansion due to increases in temperature, while the piston is nonetheless guided in a problem-free manner via the longitudinal support webs.
In some circumstances, the provision of only one revolving web protruding from the peripheral surface for the fixing of the collar-shaped seal may not be sufficient, so that it is desirable to provide two revolving webs, one at each end, that are connected by the longitudinal support webs that run parallel to one another and at a distance from one another. However, the powder-metallurgical production of such a piston body by pressing a green compact from a sinterable metal powder, with subsequent sintering, presents significant forming problems, so that in DE-A-101 08 246 it was proposed to divide the piston body into two sub-bodies, each having at one end a revolving web from which the longitudinal support webs situated at a distance from one another emanate. The two sub-bodies can be formed without great difficulty from a sinterable metal powder as green compacts, and then sintered. The complete piston is then assembled from the two finally sintered parts in such a way that the revolving webs are each situated at an end of the piston facing away from the dividing plane. By beveling the support webs at their end facing the dividing plane, it is then possible to provide an additional transverse groove. The disadvantage of this design is that the joining process for the two sub-bodies is expensive, if these have to be joined with a defined position to one another. This is for example the case for piston bodies for shock absorber pistons, because here the partial channels present in the two sub-bodies must be precisely aligned with one another.
The present invention is based on the object of creating a piston that is simple to manufacture, in particular a shock absorber piston.